Radar level gauge systems are in wide use for measuring process variables of a product contained in a tank, such as filling level, temperature, pressure etc. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged vertically from top to bottom of the tank. The electromagnetic signals are subsequently reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and receipt of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity along the probe of the electromagnetic signals.
However, the electromagnetic signal transmitted by the transceiver along the probe is typically not only reflected at the impedance transition constituted by the interface between atmosphere and surface, but at several other impedance transitions along the probe. Such impedance transitions may, for example, result from product residue that may have adhered to the probe as the filling level of the product changes inside the tank.
There is therefore a risk that the system attempts to determine the filling level based on an erroneous reflected signal.
Moreover, the reflected signal resulting from reflection at the surface of the product may under some conditions be intermittent, which may result in an unreliable filling level determination.
In order to improve the reliability of the filling level determination, US 2006/0052954 and DE 10 2004 052 110 disclose methods and systems for determining which received echo signals are surface echo signals resulting from reflection at the surface of the product in the tank, and which received echo signals are parasitic echo signals. According to each of the methods disclosed in these documents, the received echo signals are classified into surface echo candidates and parasitic echo candidates based on their respective movement over time—non-stationary echoes are classified as surface echoes, while stationary echoes are classified as parasitic echoes.
Neither of the systems and methods disclosed in the above documents provide any indication of the reliability of the level determination.